The present invention relates to the field of instruments used to measure the reflective color of an object, such as a reflection densitometer or a spectral reflectometer. These types of instruments are used extensively in measuring printed ink colors, paint colors, or the colors of any other object for which a numerical color value is desired. In the case of printing devices, the measured values are often used to calibrate the printing device in a feedback loop so as to reproduce a desired color. In other cases, the measured value is used to specify a desired color in a digital image or document, often for the purpose of matching the color of the measured object.
Instruments of this type are well known in the prior art and are generally comprised of one or more light sources and one or more light sensitive detectors arranged in specific geometry. Under common standards (such as ANSI/ISO May 4, 1995 Density Measurementsxe2x80x94Part 4: Geometric Conditions for Reflection Density, or DIN 16536 Color Density Measurements on Prints: Requirements on Measuring Apparatus for Reflection Densitometers), the measurement geometry is specified to consist of an annular ring of illumination projected onto the center of a sample target area at an angle of between 40 and 50 degrees. The light reflected off the sample is sensed by a detector positioned at an angle of between 85 and 90 degrees from the target sample. Alternatively, the positions of the light source and detector may be interchanged.
Detailed discussions relating to the background of color measurement instruments may be found in prior art such as U.S. Pat. Nos. 5,015,098 and 5,073,028.
Such measuring devices may also be configured as hand-held devices. Hand-held measuring devices used to measure the reflective color of an object are generally manually aimed by positioning a small (3 to 7 mm diameter) sampling aperture in contact with and over the area of the sample for which the color is to be measured. The manual aiming process can be tedious and error-prone due to the inability of the operator to accurately see where the sampling aperture of the measuring device is about to be placed on the sample. The enclosure of the measuring device is generally constrained by the measurement geometry requirements to taper away from the sampling aperture at an angle no greater than 40 degrees. Such a configuration significantly obstructs the operator""s sight line during aiming and positioning of the sampling aperture over an area to be sampled.
For example, a typical prior art hand-held measuring device is illustrated in FIG. 1. FIG. 1 shows a prior art measuring device 100 having a sample area optical enclosure 101. The optical enclosure 101 includes light sources 120 and 121 arranged such that light is projected towards a sampling aperture 110 at an angle of 45 degrees. The detector 130 is arranged to detect light reflected at a surface normal to the sampling aperture 110. FIG. 1 shows the measuring device 100 positioned over a sample area 210 of a sample 200. The portion of the optical enclosure 101 which tapers down to form a sampling aperture 110 is formed by the optical enclosure walls 140 which narrow toward the sampling aperture in the shape of a cone. The cone shaped walls 140 are symmetrically arranged and angled at approximately 45 degrees. This angle of the walls 140 is constrained both by the placement of the light sources 120 and 121 so as to project light toward the sampling aperture 110 at an angle of 45 degrees and by the placement of the detector 130 so as to detect light reflected at a surface normal to the sampling aperture 110.
The geometry of the prior art measuring device 100 conforms to standard measurement geometry relating to placement of the light source and detector. As can be seen from FIG. 1, such a configuration results in the optical enclosure obstructing the view of the area to be sampled. The operator sight line, shown in FIG. 1 as 300, is angled at approximately 45 degrees from the targeted sample area. Such an obstructed sight line makes it extremely difficult for an operator to accurately position the measuring device 100 over the sample area, resulting in erroneous measurements.
A slight improvement over the prior art is illustrated by FIG. 2, which shows a hand-held measuring device such as that disclosed in U.S. Pat. No. 5,963,333. FIG. 2 shows the same elements as disclosed in FIG. 1 and all reference numbers correspond.
The prior art measuring device of FIG. 2 allows for a different configuration of the light sources 120 and 121, which results in a slightly improved operator sight line 300. The light sources 120 and 121 are positioned such that the light is projected onto reflective surfaces 142 of walls 140 proximal to the sampling aperture, which reflective surfaces reflect the light towards the sampling aperture at an angle of approximately 45 degrees. Such a configuration maintains the standard measurement geometry, while allowing the optical enclosure walls 140 to be arranged symmetrically and angled at approximately 60 degrees. As shown in FIG. 2, such an arrangement of the optical enclosure walls 140 allows for a slightly improved operator sight line 300 of approximately 60 degrees.
While the sight line of the measuring device illustrated in FIG. 2 is an improvement over that of FIG. 1, such a configuration still results in an obstructed view of the area to be sampled and results in difficulty in accurately positioning the sampling aperture over the sample area. The sight line of approximately 60 degrees is less than optimal.
An operator sight line approaching the optimal 90 degrees is desired and is accomplished by the present invention.
The present invention is intended to eliminate the problems associated with the restricted sight line of the prior art color measurement instruments such as reflection densitometers or spectral reflectometers. The object of the present invention is accomplished by arranging the light source and detector in a manner which allows the sample area optical enclosure walls to taper away from the sampling aperture in an asymmetrical manner. The taper angle of the optical enclosure is higher at the front of the optical enclosure where the operator requires a better sight line, while the taper angle of the optical enclosure walls at the portions towards the rear of the optical enclosure is approximately 40 degrees. Such a configuration allows for the standard measurement geometry between the detector and sample of approximately 90 degrees and between the light source and sample of approximately 45 degrees. The asymmetrical configuration of the optical enclosure allows for an operator sight line of approximately 80 degrees.
The light source can comprise, for example, one or more incandescent light sources, one or more infrared light sources, one or more light emitting diodes, or the like. Similarly, the detector can comprise, for example, one or more detectors or series of detectors, a detector with a multitude of detection elements, or the like.
In one embodiment of the present invention, the light source is positioned only towards the rear of the optical enclosure. As light is projected into the sampling aperture from only one position, such a configuration provides illumination of the sample area that is not completely in adherence with standard measurement techniques, such as the American National Standards Institute (ANSI) standard (ANSI/ISO May 4, 1995). However, the measurements achieved are sufficiently accurate for most applications. Only in the case of heavily textured sample surfaces would the orientation of illumination become significant, and only where the textured sample is significantly directional in its arrangement.
In another embodiment of the invention, a first light source is positioned towards the rear of the optical enclosure and illuminates the sampling aperture directly at an angle of projection of approximately 45 degrees. A second light source is positioned toward the rear of the optical enclosure and projects light onto a reflective surface proximal to the front of the sampling aperture, which reflecting surface is arranged such that the light from the light source is projected into the sampling aperture at an angle of approximately 45 degrees. Such a configuration maintains the standard measurement geometry and conforms to measurement standards, while allowing the operator an improved sight line for placement of the sampling aperture over the area to be sampled. As with the previously described embodiment, the operator sight line in such a configuration is approximately 80 degrees.
Alternatively, a single light source can be positioned at the rear of the optical enclosure such that it directs light both directly into the sampling aperture at an angle of 45 degrees and at the reflective surface proximal to the front of the sampling aperture, from which the light is reflected into the sampling aperture at approximately 45 degrees.
In another embodiment of the invention, the optical enclosure comprises an outer cone and an inner cone. The inner cone acts to maximize the transmission of light reflected off of a sample to the detector. The interior surface of the inner cone may comprise a non-reflective surface for maximizing light transferred from the sample to the detector.
In a preferred embodiment the optical enclosure forms part of a hand-held measuring device such as a hand-held reflection densitometer or a hand-held spectral reflectometer which can be manually positioned over the object to be sampled.